1. Field of the Invention
The present invention relates to a substrate processing method and a substrate processing unit for processing the front surface of a substrate with a process gas.
2. Description of the Related Art
A mask is used to form a desired pattern on the front surface of a semiconductor wafer (hereinafter referred to as wafer) or a glass substrate of a liquid crystal display (the glass substrate is hereinafter referred to as LCD substrate). Such a mask is formed by coating a resist on the front surface of a wafer or the like, radiating a light beam, an electron beam, or an ion beam to the front surface of the resist, and developing the resist. To prevent the resist mask from peeling off the substrate in a developing process, an ion implanting process, and an etching process, the adhesion between the substrate and the resist should be improved. To prevent such a problem, before resist solution is coated on the front surface of a substrate, a hydrophobic process is performed on the front surface of the substrate.
A hydrophobic processing unit that performs the hydrophobic process has an airtight vessel that holds and heats a wafer. While process gas that contains HMDS (hexamethyldisilane) gas is being supplied to the airtight vessel, the airtight vessel is exhausted. When a wafer is loaded to the airtight vessel, the process gas is supplied thereto successively for around 20 seconds. At the same time, the airtight vessel is exhausted.
For example, the airtight vessel has a gas supply inlet through which the process gas is supplied. The gas supply inlet is disposed above the center portion of the wafer held in the airtight vessel. In addition, an exhaust outlet is disposed outside the periphery of the wafer. Thus, the process gas enters the airtight vessel from the gas supply inlet, hits the center portion of the wafer, flows to the periphery of the wafer along the front surface thereof, and exits from the exhaust opening.
Alternatively, the gas supply inlet and the exhaust outlet may be disposed at the reverse positions of the above-described airtight vessel. In other words, the process gas is supplied from the periphery of the wafer and exhausted from an upper position of the center portion of the wafer.
As another alternative method, the gas supply inlet and the exhaust outlet may be oppositely disposed. In this case, the process gas flows along the front surface of the wafer and exits from the exhaust outlet.
However, since the temperature of the process gas such as the HMDS gas that enters into the airtight vessel is lower than the temperature of the wafer, when the process gas enters the airtight vessel from the gas supply inlet and hits the center portion of the wafer, the center portion of the wafer is more cooled than the periphery thereof. Likewise, when the process gas enters the airtight vessel from a position in the vicinity of the periphery of the wafer and exits from an upper position of the center portion of the wafer, the periphery of the wafer is more cooled than the center portion thereof. In addition, when the process gas enters the airtight vessel from a side portion thereof and exits from the opposite side portion thereof, the wafer portion on the gas supply inlet side is more cooled than the wafer portion on the exhaust outlet side.
In any type of the above-described units, a temperature difference takes place along the direction of the flow of the process gas. Thus, the temperature distribution on the surface of the wafer W becomes unequal. For example, in the hydrophobic process using HMDS gas, the hydrophobicity on the front surface of the wafer is proportional to the process temperature of the wafer. Thus, in any type of the above-described units, the hydrophobicity of the wafer portion in the vicinity of the gas supply inlet deteriorates.
As an index that represents the hydrophobicity (or hydrophilicity), the contact angle of a water drop on the front surface of the wafer W is used. In any type of the above-described units, the contact angle on the wafer that has been hydrophobicity-processed deviates for around 3 to 4 degrees.